Within the last few years, the advantages of the integrated services digital network (ISDN) have become well-known. Among other things, the increased speed of the communications link, as well as the ability to carry on both voice and data communications simultaneously, have offered substantial advantages to the consumer.
To access ISDN services, a terminal adapter is required. A number of manufacturers currently offer such devices, but all such devices to date tend to be substantially more expensive than analog modems. Part of the expense associated with a typical ISDN terminal adapter involves the use of a relatively high performance general purpose microprocessor, together with other logic external to the microprocessor to maintain feature compatibility with the digital network.
A typical prior art terminal adapter is shown in FIG. 1, and includes a general purpose microprocessor 10, such as a Motorola 68302, which communicates bidirectionally with a plurality of external logic devices such as CODEC logic 12, DTMF RX logic 14, V.110 compatibility logic 16, and HDLC logic for the D and dual B channels, identified at 18, 20 and 22. The CODEC logic 12 and DTMF transceiver logic 14 in turn communicate with a subscriber loop interface 24, so that the system may communicate with conventional telephone sets, sometimes referred to herein as POTS1 and POTS2, or "plain old telephone set," and identified at 26 and 28.
The V.110 logic 16 and HDLC logic 18-22 communicate with standard ISDN interface logic 32, which connects to the ISDN network provided by the telephone company.
It will be appreciated that this logic is both complicated and expensive. In addition, in the event of any change in features, a typical prior art device is obsoleted because the design is substantially hardware-based and therefore offers little flexibility for upgrading. There has therefore been a need for a design of ISDN terminal adapter which minimizes costs while offering both full functionality and the flexibility to incorporate new features.